Minimally invasive procedures: Minimally-invasive diagnostic or interventional procedures require either direct visual viewing or indirect imaging of the field of operation and determination of the location and orientation of the operational device. For example, laparoscopic interventions are controlled by direct viewing of the operational field with rigid endoscopes, while flexible endoscopes are commonly used for diagnostic and interventional procedures within the gastro-intestinal tract. Vascular catheters are manipulated and maneuvered by the operator, with real-time X-ray imaging to present the catheter location and orientation. Ultrasound imaging and new real-time MRI and CT scanners are used to guide diagnostic procedures (e.g. aspiration and biopsy) and therapeutic interventions (e.g. ablation, local drug delivery) with deep targets. While the previous examples provide either direct (optical) or indirect (imaging) view of the operation field and the device, another approach is based on remote sensing of the device with mechanical, optical or electromagnetic means to determine the location and orientation of the device inside the body.
Stereotaxis: Computer-assisted stereotaxis is a valuable technique for performing diagnostic and interventional procedures, most typically with the brain. The concept behind the technique is to have real-time measurement of the device location in the same coordinate system as an image of the field of operation. The current location of the device and its future path are presented in real-time on the image and provide the operator with feed-back to manipulate the device with minimal damage to the organs. During traditional stereotaxis, the patient wears a special halo-like headframe, which provides the common coordinate system, and CT or MRI scans are performed to create a three-dimensional computer image that provides the exact location of the target (e.g. tumour) in relation to the headframe. The device is mechanically attached to the frame and sensors provide its location in relation to the head frame. When this technique is used for biopsy or minimally-invasive surgery of the brain, it guides the surgeon in determining where to make a small hole in the skull to reach the target. Newer technology is the frameless technique, using a navigational wand without the headframe (e.g. Nitin Patel and David Sandeman, “A Simple Trajectory Guidance Device that Assists Freehand and Interactive Image Guided Biopsy of Small Deep Intracranial Targets”, Comp Aid Surg 2:186-192, 1997). In this technique remote sensing system (e.g. light sources and sensors) provides the real-time location of the device with respect to the image coordinate system. Yet both the stereotaxis and the frameless techniques are typically limited to the use of rigid devices like needles or biopsy forceps since their adequate operation requires either mechanical attachments or line of sight between the light sources and the sensors.
Electromagnetic remote sensing: Newer remote sensing techniques are based on electromagnetism. For example, the Bladen and Anderson technique (WO 94/04938) is an active electromagnetic tracking methodology that requires the use of electromagnetic field generator or generators to determine the location and orientation of a sensor (Page 3 lines 17-36; Page 4 line 24 through Page 5 line 9). This methodology cannot be directly used during MRI because the application of an external electromagnetic field creates an unacceptable level of image artifacts. These artifacts can be avoided by interleaving the tracking step with the image acquisition step, which requires a modification of the MRI pulse sequences and lengthens the imaging time. It also requires mechanical modification of the scanner, to add the field generators into the structure of the scanner. The addition of conducting elements to the scanner (the coils of the generators) may result in substantial artifacts due to the creation of eddy currents and may create electromagnetic interference with the scanner. Acker et al (U.S. Pat. No. 5,558,091) disclose such a method and apparatus to determine the position and orientation of a device inside the body. This method uses magnetic fields generated by Helmholtz coils, and a set of orthogonal sensors to measure components of these fields and to determine the position and orientation from these measurements. The measurement of the magnetic field components is based on Hall effect and requires exciting currents in the sensors in order to generate the measured signals. The technique requires control of the external magnetic fields and either steady-state or oscillating fields, for the induced voltages to reach a state of equilibrium. These requirements prevent, or greatly complicate, the use of this technique with magnetic fields generated by the MRI system, and requires the addition of a dedicated set of coils to generate the required magnetic fields.
A different approach for remote sensing of location is disclosed by Pfeiler et al. (U.S. Pat. No. 5,042,486) and is further used by Ben-Haim for intra-body mapping (U.S. Pat. No. 5,391,199). Their technology is based on generating weak radio-frequency (RF) signals from three different transmitters, receiving the signals through an RF antenna inside the device, and calculating the distances from the transmitters, which define the spatial location of the device. As with the previous methodology, the application of the technology to MRI is problematic due to the simultaneous use of RF signals by the MR scanning. Potential difficulties are the heating of the receiving antenna in the device by the high amplitude excitation RF transmissions of the MRI scanner and artifacts in the MR image.
Dumoulin and colleagues disclose another approach to determine the location of a device, using a small receiving coil which is sensitive to near-neighborhood emitted RF signal during the MR imaging process (Dumoulin C L, Darro R D, Souza S P, “Magnetic resonance tracking”, in Interventional MR, edited by Jolesz F A and Young I Y, Mosby, 1998; U.S. Pat. No. 5,318,025). Dumoulin and colleagues (U.S. Pat. No. 5,318,025) teach a tracking system based on induction of signals in receiving coils by the radio-frequency (RF) emission from the tissue near the device (Column 4, lines 33-50). It requires the use of all the components of an MRI scanner, including a high-intensity homogenous magnetic field, a set of three orthogonal magnetic gradient fields, RF transmission system, an RF receiving system, in addition to its own processing and controlling modules (FIG. 6 and Column 6, line 44 through Column 7, line 2). Since it is based on the same mechanism as that of image acquisition by MRI scanner, it can be viewed as imaging of a very small region of interest. Consequently, it requires the presence of material that can generate MR signals in the vicinity of the RF coils, either tissue (if the sensor is inside the body, like in catheters) or a small chamber of contrast agent (if tracking is done outside the body). Furthermore, the Dumoulin system requires substantial modification of the normal sequence of image acquisition to enable tracking (FIG. 4 and Column 4). This substantially complicates the programming of the pulse sequence of the scanner (due to the need to add RF and gradient activations for tracking that interfere with the imaging sequence), lengthens the time of scan, and limits the update rate of tracking. Additional major limitations of this tracking technique include the potential heating of the RF coils, especially inside the body; and the need to determine the orientation indirectly through the use of estimated location of at least 2 separate RF coils, with limited accuracy when small sensor must be used (i.e. when the distance between the two coils is short).
Interventional MRI: Many of the advantages of MRI that make it a powerful clinical imaging tool are also valuable during interventional procedures. The lack of ionizing radiation and the oblique and multi-planar imaging capabilities are particularly useful during invasive procedures. The absence of beam-hardening artifacts from bone allows complex approaches to anatomic regions that may be difficult or impossible with other imaging techniques such as conventional CT. Perhaps the greatest advantage of MRI is the superior soft-tissue contrast resolution, which allows early and sensitive detection of tissue changes during interventional procedures. Many experts now consider MRI to be one of the most powerful imaging techniques to guide interventional interstitial procedures, and in some cases even endovascular or endoluminal procedures (Yoshimi Anzai, Rex Hamilton, Shantanu Sinha, Antonio DeSalles, Keith Black, Robert Lufkin, “Interventional MRI for Head and Neck Cancer and Other Applications”, Advances in Oncology, May 1995, Vol 11 No. 2). From the presented background on current methodologies, one can define the ideal system for minimal invasive procedures: It should provide real-time, 3-dimensional, non-ionizing imaging (like MRI or ultrasound) as feed-back to the user for optimal insertion and intervention; it should implement flexible, miniaturized devices which are remotely sensed to provide their location and orientation. By combining a composite image of the field of operation and the device location and orientation, the operator can navigate and manipulate the device without direct vision of the field of operation and the device. This may facilitate the use of minimal invasive intervention in the brain or other organs.
Motion artifacts in diagnostic MRI: Motion during scan hampers the image quality of MRI. The relative phase evolution of the MR signal induced as a result of the nuclear spin motion between the phase encode steps, presents as ghosting and smearing of signal intensity to incorrect locations in the image space (A. W. Anderson, J. C. Gore, “Analysis and correction of motion artifacts in diffusion weighted imaging,” Magn. Reson. Med., vol. 32, pp. 379-387, 1994; R. J. Ordidge, J. A. Helpern, Z. X. Qing, R. A. Knight, and V. Nagesh, “Correction of motional artifacts in diffusion weighted MR images using navigator echoes,” Magn. Reson. Med., vol. 12, pp. 455-460, 1994). Motion induced artifacts, specially the ghost and blurring artifacts, can significantly degrade the imaging sequences and also severely reduce the accuracy of functional MRI by deteriorating the weak activation signal. Retrospective motion correction techniques are currently used in routine MRI applications (M. Jenkinson, P. Bannister, J. M. Brady, S. M. Smith, “Improved optimization for the robust and accurate linear registration and motion correction of brain images,” NeuroImage, vol. 17, pp. 825-841, 2002; D. Atkinson, D. L. G. Hill, P. N. R. Stoyle, P. E. Summers, S. Clare, R. Bowtell, and S. F. Keevil, “Automatic compensation of motion artifacts in MRI,” Magn. Reson. Med., vol. 41, pp. 163-170, 1999; B. Kim, J. L. Boes, P. H. Bland, T. L. Chenevert, C. R. Meyer, “Motion correction in fMRI via registration of individual slices into an anatomical volume,” Magn. Reson. Med., vol. 41, pp. 964-972, 1999; V. L. Morgan, D. R. Pickens, S. L. Hartmann, R. R. Price, “Comparison of functional MRI image realignment tools using a computer-generated phantom,” Magn. Reson. Med., vol. 46, pp. 510-514, 2001; L. Freire, and J. F. Mangin, “Motion correction algorithms may create spurious brain activations in the absence of subject motion,” Neuroimage, vol. 14, pp. 709-722, 2001). The realtime tracking of motion, for example by a tracking sensor that is attached to the moving organ, can be used prospectively during the scan to eliminate, or reduce, the effect of motion on the image quality.
MRI Pulse Sequence
A pulse sequence is a pre-selected set of defined radiofrequency (RF) and gradient pulses, usually repeated many times during a scan, wherein the time interval between pulses and the amplitude and shape of the gradient waveforms will control the MR signal reception and affect the characteristics of the MR images. Pulse sequences are specialized computer programs that control all hardware aspects of the MRI scan. Specific pulse sequences have been created to achieve various contrast mechanisms in the obtained images, and they are typically dependent on the magnetic field strength, the characteristics of the gradient fields, the characteristics of the RF fields, the type of imaging coil used, the clinical application of the sequence, and more.
Gradient Based Tracking
The EndoScout tracking system (Robin Medical, Baltimore, Md.) was developed to assist MRI-guided interventions in accordance with methods disclosed in U.S. Pat. No. 6,516,213, incorporated herein by reference in its entirety. The tracking is based on the gradient fields of the scanner. The system passively “listens” to the gradient activations and can track any device without the need to generate reference fields (as done by virtually all other electromagnetic tracking technologies). The EndoScout provides in real-time the position (location and orientation) of the device during MR scanning. This data enables the users to guide an interventional procedure to a target inside the body, using a minimally invasive approach. Devices and tools to be tracked may include: diagnostic devices (e.g. biopsy needles, aspiration needles), therapeutic devices (including devices used to destroy tissue, like RF ablation, laser ablation or cryo-therapy probes, or devices used to deliver therapeutic agents like drugs, chemotherapy agents, genetic vectors, stem cells, etc.) and various support tools (e.g. suction tips, pointers).
The main advantage of the gradient-based tracking technology is its independence of scanner type and mode of operation. However, some features of the pulse sequences may have adverse effects on the tracking performance and may result in less accurate or non-stable tracking. For example, the creation of 2-dimensional images requires encoding in two directions, which is achieved by frequency encoding and by phase encoding. The phase encoding typically involves gradient activations with variable amplitudes, beginning with a high positive amplitude, progressing through a low level and zero level amplitude, and ending with a high negative amplitude. During the time when gradient activations go through the low amplitude range, the tracking may be less accurate and create fluctuations in the tracking results that are in phase with the phase encoding of the scanning process. To eliminate this inaccuracy, the sampling window of the tracking system can be enlarged. To achieve maximal accuracy and stability, the sampling interval should include the whole length of the phase encoding cycle. While this can eliminate the accuracy problem, it may also result in low dynamic response of the tracking system, as typically large sampling windows are used, and the tracking results may be inaccurate if the tracked object is moving.
Another potential problem is the existence of eddy currents, which are induced when the gradient fields are changed over time, i.e. during activation and deactivation. These eddy currents are a well known phenomenon in MRI technology and are accounted for by the imaging process. The EndoScout tracking system uses maps that describe the spatial distribution of the gradient magnetic fields in each scanner where the system is used. These gradient field maps are measured during installation of the system. Eddy currents constitute a complex non-linear phenomenon that may depend on the specific gradient activation pattern in each moment, so eddy currents that affect the gradient field during mapping may differ from eddy currents that affect the gradient field during scan. Thus it may be difficult or even impossible to account for all types of eddy currents that are generated by various gradient combinations that are used during a scan.